An arc-welding method is known, which comprises, when welding the objects of welding together in the longitudinal direction of a groove formed between the objects of welding by the arc-welding method using a consumable or non-consumable welding electrode, reciprocally moving a welding torch, following the face of the groove, in the width direction of said groove, and, at the same time, continuously moving said torch in the longitudinal direction of said groove, to automatically weld the objects of welding together in the longitudinal direction of the groove.
As a welding method as mentioned above, we proposed an arc-welding method as disclosed in Japanese Patent Provisional Publication No. 19,445/79 dated Feb. 14, 1979 (hereinafter referred to as the "prior art"). The prior art is a method which comprises detecting the position of the welding electrode in a groove by means of arc current or arc voltage of an arc produced between the welding electrode and the groove, and welding the objects of welding together while controlling the position of the welding electrode on the basis of the result of detection, as summarized as follows:
The method of the prior art comprises: continuously moving a carriage along the longitudinal direction of a groove formed between the objects of welding, fitting a welding torch to said carriage so that said torch is directed substantially vertically to said groove, said torch moving together with said carriage; directing a welding electrode through said torch toward said groove; feeding a welding current to said electrode to produce an arc between the tip of said electrode and said groove to weld said objects of welding together by means of the arc heat along said groove; continuously moving said torch, during travel of said carriage, in the width direction of said groove while continuing welding; continuously detecting a value (E.sub.v) of one arc voltage and arc current of said arc; continuously calculating a deviation (E.sub.o -E.sub.v) of the thus detected value (E.sub.v) from a previously set value (E.sub.o) of one of arc voltage and arc current; continuously moving said torch vertically up and down, during travel of said carriage, so that said deviation (E.sub.o -E.sub.v) becomes null, thereby maintaining the distance between the tip of said electrode and said groove always at a prescribed distance (l) during the movement of said torch in the width direction of said groove; continuously detecting a vertical position of said torch as a value of voltage (e.sub.Y) indicated by a potentiometer for each movement of said torch in the with direction of said groove; reversing the direction of movement of said torch in the width direction of said groove at the moment when said value of voltage (e.sub.Y) thus detected agrees with a previously set value of voltage (e.sub.H); repeating said movement of said torch in the width direction of said groove, said movement of said torch in the vertical direction and said reversal of the direction of the movement of said torch in the width direction of said groove; on the other hand, continuously moving said torch in the longitudinal direction of said groove, while continuing welding, together with said carriage; thereby reciprocally moving said torch in the width direction of said groove following the face of said groove while maintaining the distance between the tip of said electrode and said groove always at said prescribed distance (l), and, at the same time, moving said torch in the longitudinal direction of said groove so as to weld said objects of welding in the longitudinal direction of said groove.
According to the prior art presented above, since it is possible to detect the position of the welding electrode in a groove by means of arc current or arc voltage and carry out welding while moving the welding torch reciprocally in the width direction of the groove, thus permitting welding, accurately following the welding line.
However, the above-mentioned prior art was found to involve the following problems. Even if the objects of welding have flat surfaces, rails for travelling the carriage on which a welding machine is mounted, laid in parallel with the longitudinal direction of the groove on the object of welding may sometimes curve up or down relative to the objects of welding, or even if the rails are laid straight, the objects of welding may sometimes curve up or down. According to the prior art, reversal in the reciprocating motion of the welding torch in the width direction of the groove is effected at the moment when the detected value of voltage representing the position of the welding electrode agrees with a certain value of voltage indicating the reversing position of the torch which has previously been set on the assumption that the distance between said carriage and the objects of welding is kept at a previously set distance. Therefore, when the distance between said carriage and the objects of welding deviates from the previously set distance under the effect of curved rails or curved objects of welding as mentioned above, the moving direction of the torch is reversed at a point other than a previously set position of reversal. This results in a formed bead becoming irregular in height, a defective penetration and production of welding defects, thus making it impossible to obtain a sound weld.